The present invention pertains to quadraphase shift keyed modulators (QPSK) and biphase shift keyed modulators (BPSK) and more particularly to improvements to minimizing errors for data transmission utilizing QPSK and BPSK demodulators.
In transmission systems, signal conditioning for high performance and low signal energy-to-noise power density ratio have typically been treated as of secondary importance. This leads to severe carrier loop parameter variations. Loop gain variations may approach 30 dB.
One QPSK demodulator circuit is shown in U.S. Pat. No. 4,085,378, issued on Apr. 18, 1978 to C. Ryan et al. and assigned to the same assignee as the present application. This patent teaches improvements to the Costas loop demodulator. This patent teaches providing a plurality of adjusting signals for tracking the phase error of the I and Q channels of the demodulator. As a result, much adjusting interconnection must be provided. In addition, the teachings of this patent provide no indication of the production of signals proportional to the amplitude of the data signals.
In addition, U.S. Pat. No. 4,053,837, issued on Oct. 11, 1977 to C. Ryan et al. and assigned to the same assignee as the present invention, teaches an equalizer arrangement for a QPSK demodulator system. Again a plurality of adjustments are derived and no teachings are made as to the amplitude signal production.
Neither reference teaches circuitry which is capable of both QPSK nd BPSK demodulation with minor variations.
Accordingly, it is an object of the present invention to reduce the loop gain significantly for a QPSK and BPSK demodulator while providing minimal control signals and amplitude signals as outputs.